Smooth film surfaces tend to ‘block’ or stick together when stacked or rolled. This is particularly troublesome when rolled substrates are stored at high temperatures and humidity. Anti-blocking or slip agents have long been known to provide surface roughness to prevent adhesion between two sheets of what would otherwise be smooth film surfaces. The effect of roughening the surface is to reduce the frictional forces between the surfaces of sheets or layers of the substrate. Many inorganic and polymeric materials are known to act as good anti-blocking agents and various solutions to the problem have been proposed. Unfortunately, when particulates are added or surface roughness increased, generally, the film haze or light scattering properties of the films are also increased.
Another acceptable way to overcome blocking or high surface friction in rolled films is to utilize a variable knurling profile in the wound roll as described in U.S. Pat. No. 5,393,589. Knurling refers to the processes for producing a pattern of raised features on one or both sides of a web. In variable knurling, the height of the raised features is varied throughout the length of the roll in a predetermined profile to allow for high knurl heights near the core where pressure damage can occur and thinner knurl heights at the outer layers of the roll where distortion in the web can occur.
In coating anti-blocking agents on substrates used in optical devices, improved surface slip must be achieved without any deterioration of the optical properties. An example of this situation would be the use of anti-blocking layers on triacetyl cellulose (TAC) film as more fully described later. TAC film is typically used as polarizer protective layers in the manufacture of LCD's. The fundamental lack of TAC polymer orientation combined with the low stresses of solvent casting, forms a unique polymer system for extremely isotropic LCD coversheets. These fundamental advantages have allowed solvent cast cellulose triacetate to capture the vast majority of LCD coversheet applications. However, the TAC is a soft film and when produced and rolled, the smooth front and back film surfaces have a tendency to stick or block together and generate poor wound roll quality which leads to defects in the LCD protective layers.
The typical method, which has been used to provide modified surface friction and anti-blocking properties to TAC films out in the industry, has been to incorporate fine silica particles (10-20 nm) into the cast film. However, surface only application of the matting particle is preferred as this minimizes the amount of material to be incorporated. Also for good optical properties as well as anti-blocking properties polymeric beads are often advantageous. A bead type can be chosen for the functionality of the components, hardness (usually expressed as degree of crosslinking), size and narrow particle size distribution. For example, U.S. Pat. No. 5,238,736 discloses the combination of the hardness of polymethylmethacrylate with the lubricity of long-chain hydrocarbons in microspheres produced from homopolymers of diol di(meth)acrylates and/or copolymers with long-chain aliphatic alcohol esters of (meth)acrylic acid and/or ethylenically unsaturated comonomer. This type of bead is demonstrated to work well for smooth hard coated layers such as acrylic coatings on PET. The patent specifically teaches that there is a minimum in the chain length for the diol di(meth)acrylate in order for advantaged friction properties to be observed and there are no teachings to describe the impact of a coated matte layer on the optical property requirements of films.
In the case of coating on a soft substrate such as TAC the degree of crosslinking in the matte beads is surprisingly not a sufficient measure of hardness to be able to provide good surface friction properties which are important to predicting good wound roll quality. A better indicator of good surface friction performance is the degree to which the bead swells in a coating solvent.